Resonance is pretty darn good. I would look for a single magic number for all instances of it. (Probably 3)
Another possibility is "as long as you control no permanents with colors that don't match this card's color"

Fascist Crusader 1WW
2/2 Knight
First strike
Resonance — ~ has protection from nonwhite as long as you control no nonwhite permanents.

You rarely want two mechanics in the same set that are completely oppositional to each other. Ignoring that, Dissonance is going to lead to a lot of frustration when players inevitably draw a second card of a color and don't want to cast it.

Prism Man is awesome (though undercosted), but Prism Land terrifies me. Not only is it reminiscent of the original Mirrodin artifact lands, but it's the first land to have any color at all and this could confuse players into thinking that lands do have colors.

My only hesitation in setting the number at three is that it's essentially Colorcraft at that point. Could you scale the number along rarity lines?

Then again, that's more of a development concern than a design concern.

Re: Prism Man - If Transguild Courier is a 3/3 for 4, I think a 2/2 for 2 is perfectly acceptable.

Prism Land should likely be legendary. I envisioned it as the Rare topper to an uncommon cycle of color lands. Although, to avoid the confusion - the lands could be the color / colors of their allied color.

I feel that Resonance is too restrictive; it would take away people's ability to combine colors so there will be less variety in deck-building and drafting. I much prefer the style that the (2/C) costs use - they natural encourage mono-color in a way that's natural and not restrictive, and they can also be used by multicolor decks.

Something like color-fall from Shadowmoore would be less restrictive than resonance while still rewarding mono-color decks.

Generally, mono-color would mean that you only get 5 main archetypes in the set. I wouldn't want a mono-color theme unless there is also plenty of reasons to play 2+ color decks. Also, I think mono-color decks should be diversified by the splash colors they choose. A B/r deck should be allowed to behave differently from a B/u deck. So, whatever implementation is chosen, the mechanic should leave some room open for splashing.

Just to clarify - it's only restrictive if you go with the alternative "each colored permanent you control is white." or "you control no nonwhite permanents." wording.

If you just reward the player for playing lots of red cards or playing lots of blue cards, then you don't have that problem. It actually increases the incentive for doing so - but then it really is just Color-craft.

Speaking to that effect - changing Dissonance from "as long as you control no permanents that share a color with this" to "if you control permanents of X different colors" rewards you or playing lots of different colorless hybrid cards.